The embodiments described herein relate generally to turbines, and more particularly, to methods and systems for thermal management of a turbine shaft.
Recently, the use of high power density working fluids, such as supercritical carbon dioxide (CO2) and ultrahigh pressure steam, is being explored as a potential working fluid for use with axial flow turbine engines. The use of high power density working fluids generally enables axial flow turbine engines to operate at increased rotational speeds. As such, smaller turbine engines utilizing high power density working fluids can produce similar power outputs relative to comparatively larger turbine engines utilizing conventional working fluids or at lower operating pressures. However, operating turbine engines with high power density working fluid and at increased rotational speed may present challenges relating to torque transmission, loading, inefficient thermal gradients, seal degradation, and efficiency losses of traditional mechanisms that couple turbine stages to rotor shafts.
Supercritical CO2 may have high operating temperatures and high heat transfer coefficients which may result in a high Biot number. The Biot number is an index of the ratio of the heat transfer resistances within and at the surface of a heated component such as, for example, a turbine shaft and a turbine stator. The ratio can determine whether or not the temperatures within the heated component will vary significantly while the component heats or cools over time with respect to a thermal gradient of the heated component. Typically, thermal gradients with small Biot numbers such as, for example, numbers less than one are thermally manageable. Biot numbers larger than one, however, may indicate thermal gradient problems due to non-uniformity of temperature fields created within the heated component.
Supercritical CO2, as a working fluid in a turbine, may result in a Biot number larger than one which may create excessive thermal gradients in the turbine shaft and/or stationary components of the turbine. Large thermal gradients may induce excessive axial and/or hoop stresses in turbine components which may lead to reduced component life, inefficient turbine operations, and increased operating, maintenance, and/or replacement costs for the turbine. Moreover, the high operating temperatures of supercritical CO2 may exceed temperature limits of dry gas seals present with the turbine. Consequently, supercritical CO2 may over heat the dry gas seals which may lead to efficiency losses and/or damaged turbine components.